Method of making arc extinguishing sleeve



y 25, 1965 s. M. SHOBERT: 3,184,829

METHOD OF MAKING ARC EXTINGUISHING SLEEVE Filed July 1'7, 1961 k 2Sheets-Sheet 1 ELEEL l COMPRESSED BORIC ACID HELICALLY BRAIDED 24 GLASSTHREADS RESIN I IMPREGNATED I I I I 2s 1 I I ELIE; E Ill-Th; 4

BORIC ACID /32 POWDER COMPRESSED CYLINDER OF BoRIc ACID R-CONTAINER "Jas f as |-MANDREL mmvron SAMUEL M. SHOBERT Wad f Juhlu ATTORNEYS May 21965 s. M. SHOBERT 3,184,829

METHOD OF MAKING ARC EXTINGUISHING SLEEVE Filed July 17. 1961 2Sheets-Sheet 2 HEATED MOLD RESIN INVENTOR. SAMUEL M. SHOBERT ATTORNEYSUnited States Patent 3,184,829 METHOD OF MAKING ARC EXTINGUISHING SLEEVESamuel M. Shobert, RR. 2, Ireland Road, Mishawnlra, Ind. Filed July 17,1961, Ser. No. 124,733 4 Claims. (Cl. 29-1555) This invention relatesgenerally to are extinguishing devices, and more particularly to areplaceable fuse holder and the method of making the same.

A common type of circuit interrupting device for use in electricalcircuits is the replaceable fuse. These fuses conventionally comprise areplaceable fuseable element removably inserted in a sleeve or tube ofinsulating material. In the past, such tubes have comprised an innersleeve adjacent the are formed of bone fiber with an outer covering oflaminated phenolic paper. Bone fiber has been employed since it will notsupport combustion and .yet evolves gases when exposed to an electricare which extinguish the are. Bone fiber however is too unstable topermit use by itself since it readily absorbs water and losesdimensional stability and thus the bone fiber sleeve must be supportedby some other non-conducting material. Further, the gases which areevolved from the bone fiber are confined in a relatively small spacethus generating high pressures and the bone fiber alone does not possessthe requisite strength to withstand such pressures. Additionally, bonefiber is expensive, difiicult to make and difficult to work with.

While the aforementioned fuse tube employing an inner bone fiber sleeveand an outer covering of laminated phenolic paper has been used for manyyears, it nevertheless has had relatively low bursting strength sincethe paper covering was not sufiiciently strong. However, difficultieshave been encountered in attempting to provide a different covering tobond to the bone fiber and which was sufliciently strong to overcome itsdimensional instability. It is therefore desirable to provide arefusable fuse tube having the requisite arc extinguishing propertiesand increased bursting strength which can be more economicallymanufactured than prior fuse tubes known to the present applicant.

It has been discovered that boric acid possesses very much the same areextinguishing properties as bone fiber. Boric acid in powdered form ischeap and readily available, however, in order to employ it in a fusetube, it is necessary to form it into a sleeve and to cover it with anouter covering possessing the requisite bursting strength. My inventionin its broader aspects, therefore, provides an arc confining andextinguishing sleeve comprising an inner sleeve formed of boric acidhaving glass threads braided on its outer surface, the threads beingimpregnated in hardened and cured resin thereby forming an outer sleevebonded to the inner sleeve.

It is an object of my invention to provide an improved arc extinguishingdevice.

Another object of my invention is to provide an improved refusable fusetube having arc extinguishing properties and high bursting strength.

A further object of my invention is to provide an improved method ofmaking an arc extinguishing device.

A still further object of my invention is to provide improved method ofmaking a refusable fuse tube.

Further objects and advantages of my invention will become apparent byreference to the following description and the accompanying drawing, andthe feature of novelty which characterize my invention will be pointedout with particularity in the claims annexed to and forming a part ofthis specification.

In the drawings:

FIG. 1 is a side view, partly in cross-section and partly broken away,showing a fuse incorporating the improved refusable fuse tube of myinvention; and

FIGS. 2 through 8 inclusive schematically illustrate the steps involvedin practicing the method of my invention.

Referring now to FIG. 1 of the drawing, there is shown a fuse, generallyindicated at 10, comprising an elongated inner cylindrical sleeve 12formed of compressed boric acid powder. Inner sleeve 12 has a coaxialopening 14 formed therethrough for accommodating the fusable element aswill be hereinafter more fully described. A plurality of glass threadsare helically braided on the outer surface of the sleeve 12, the threadsbeing impregnated in and the outer surface of the sleeve 12 being coatedwith hardened and cured resin to form an outer sleeve or covering 16bonded to the inner sleeve 12. The outer surface of the outer sleeve ispreferably polished.

While suitable dimensioning of the glass threads forming the braid andsuitable resins are well known in the fiber glass-resin art, I prefer toemploy threads of the glass yarn type, each thread being composed of amultiplicity of tiny elongated fibers which, in effect when bundledtogether form a final continuous length of thread. I further prefer toemploy thermo-setting resins of the polyester, polyvinyl or epoxy type.

The upper end 18 of the outer sleeve 16 is suitably threaded andinteriorly threaded metallic contact element 20 is removably securedthereto, as shown, thereby closing the upper end of the central opening14. Inserted within the central opening 14 is a replaceable fuse link 22having one end 28 electrically connected to the contact element 20 andits other end 30 connected to an electrical lead 24 which extends out ofthe other end of the fuse holder and is connected to another contactelement 26 in accordance with conventional practice. It will be readilyseen that when the contacts 20 and 26 are connected in an electricalcircuit, the fusable link forms a part of the circuit and current willflow therethrough. When a predetermined excessive current flows throughthe fusable link 22 it will be ruptured and an electrical arc will beestablished between the two terminals 28 and 30 connected to therespective ends of the fusable link 22. The presence of this are withinthe central opening 14 of the boric acid sleeve 12 will cause theevolution of arc-extinguishing gases. It will further be readily seenthat the high pressure gases which are generated within the centralopening 14 subject the inner sleeve 22 to very stresses. However, suchstresses are resisted by the helical braid and its cured resinimpregnation forming the outer covering 16 thus substantially increasingthe bursting strength of my improved fuse holder over that provided byprior fuse holders known to the present applicant.

Referring now to FIGS. 2 through 8 inclusive, in accordance with themethod of my invention, an elongated cylindrical steel mandrel 32 isdisposed coaxially within an elongated hollow cylindrical container 34and extending snugly through opening 36 in bottom wall 38 thereof, asshown. Commercial grade boric acid powder is then poured into the openend 40 of container 34 substantially to the top thereof so as tosurround the mandrel 32. It will be readily understood that the mandrel32 is proportioned to provide center opening 14 in sleeve 12 of thedesired diameter and the inner wall of container 34 is spaced from themandrel 32 by a radial distance sulficient to provide the desired wallthickness of the sleeve 12 for the requisite voltage capacity of thefinished fuse tube.

A suitable plunger 42 is then positioned over the upper end of mandrel32, as shown in FIG. 3, and is moved downwardly in the direction shownby the arrow 44 so as to compact the boric acid powder within thecontainer 34 and around the mandrel 32.

The cylinder 34 is then removed from the mandrel 32 leaving theresulting sleeve 12 of compressed boric acid on the mandrel 32, as shownin FIG. 4. A cake of compressed boric acid powder is itself fragile,however, I have found that when the boric acid powder is compressed andcompacted around the steel mandrel 32, it is self supporting and readilyhandled since the steel mandrel imparts strength to the body.

Referring now specifically to FIG. 5, the sleeve 12 of compacted boricacid powder on the mandrel 32 then has a helical braid 46 of glassthreads formed thereon by means of a conventional commercially availablebraiding machine 48. This machine comprises a rotatable supporting table50 on which a suitable number of spools 52 of glass thread or strandsare supported. Each of the spools 52 is rotatably mounted on a spindle54 which has its lower end seated in the usual serpentine or undulatinggrooves 55 of the table 50, as well known to those skilled in the art.The inner sleeve 12 on the mandrel 32 has the threads from the spools 52secured to its upper end 56 and is then moved upwardly through centeropening 58 in table 50 of braiding machine 48 in the direction shown bythe arrow 60 as the table is rotated thereby to move the spools 52 inthe grooves 55 in table 50 to form the helical braid 46. Braidingmachines and their mode of operation are well known to those skilled inthe art and thus it is unnecessary further to describe the braidingmachine 48.

After the braid 46 has been formed on the outer surface of the sleeve12, the resulting assembly is then immersed or wetted by some suitablemeans, as by being dipped in a tank 62 of suitable catalyzed resin, asshown in FIG. 6 and as well known in the glass fiber-reinforced resinart. It is important to insure that all of the strands forming the braidare thoroughly impregnated with the resin and further to insure that asufficiently thick layer of resin is formed over the braid 46 completelycoating the same and bonded to the outer surface of the sleeve 12.

The compressed boric acid sleeve 12 with the braid 46 thereonimpregnated with resin, as above described and still on the mandrel 32is then placed in a curing mold 64. This mold is heated by the use ofsteam or any other heating medium, until the resinous material iscompletely cured, the temperature and time of curing depending ofcourse, on the particular resin employed.

The resulting assembly is then removed from the mold 64, the steelmandrel 32 is removed, and the outer surface of the resulting outercovering 16 may then be ground smooth, as shown in FIG. 8.

It will be seen that with removal of the steel mandrel, the inner sleeve12 of compressed boric acid powder is provided for accommodating thefuse link 22 and it will be seen further that the provision of thehelical braid of glass fibers impregnated and bonded to each other andto the sleeve 12 stabilizes the compressed boric acid sleeve and givesit excellent dimensional stability and greatly improved burstingstrength properties.

While I have illustrated and described specific embodiments of myimproved replaceable fuse tube and method of making the same, furthermodifications and improvements will occur to those skilled in the artand I desire therefore in the appended claims to cover all modificationswhich do not depart from the spirit and scope of my invention.

What is claimed is:

l. The method of making an arc confining and extinguishing sleevecomprising the steps of: forming an elongated inner sleeve of boric acidpowder about a supporting mandrel and compressing said powder intotightly compacted form; forming a braid of glass threads on the outersurface of said sleeve; impregnating said braid with a resin material;curing said resin to bond the threads of said threads together and tosaid inner sleeve; and removing said mandrel from said inner sleeve.

2. The method of making a refusable fuse holder comprising the steps ofplacing an elongated cylindrical supporting mandrel coaxially in anelongated cylindrical mold having a closed lower end and an open upperend; pouring boric acid powder into said mold around said mandrel;compressing said powder in said mold around said mandrel to form atightly compacted inner sleeve; removing said mandrel from said moldwith said sleeve thereon forming a helical braid of glass threads on theouter surface of said inner sleeve; impregnating said braid with a resinmaterial; curing said resin to bond the threads of said threads togetherand to said inner sleeve; and removing said mandrel from said innersleeve.

3. The method of making a refusable fuse holder comprising the steps of:placing an elongated cylindrical supporting mandrel coaxially in anelongated cylindrical mold having a closed lower end and an open upperend; pouring boric acid powder into said mold around said mandrel;compressing said powder in said mold around said mandrel to form atightly compacted inner sleeve; removing said mandrel from said moldwith said sleeve thereon; forming a helical braid of glass threads onthe outer surface of said inner sleeve; dipping said mandrel with saidinner sleeve and braid thereon in a bath of resin material thereby toimpregnate said braid; placing said mandrel with said inner sleeve andimpregnated braid thereon in a heated mold to cure said resin thereby tobond said threads together and to said inner sleeve; and removing saidmandrel from said inner sleeve.

4. The method of claim 3 further comprising the steps of working theouter surface of said bonded threads to render said holder smooth.

References Cited by the Examiner UNITED STATES PATENTS 2,077,276 4/37Slepian 200-120 2,082,028 6/3'7 Rawlins 200-120 2,183,751 12/39 McMahonet al. 200-120 2,279,630 4/42 McMahon 200-149 2,328,825 9/43 McMahon200-149 2,493,434 1/50 Yonkers 29-1555 2,586,171 2/52 McMahon 200 1202,740,187 4/56 Jacobs et al. 29-1555 2,870,295 1/59 Haroldson et al.200-120 2,880,289 3/59 Baker 200-120 2,929,900 3/60 White 200-120BERNARD A. GILHEANY, Primary Examiner.

1. THE METHOD OF MAKING AN ARC CONFINING AND EXTINGUISHING SLEEVECOMPRISING THE STEPS OF: FORMING AN ELONGATED INNER SLEEVE OF BORIC ACIDPOWDER ABOUT A SUPPORTING MANDREL AND COMPRESSING SAID POWDER INTOTIGHTLY COMPACTED FORM; FORMING A BRAID OF GLASS THREADS ON THE OUTERSURFACE OF SAID SLEEVE; IMPREGNATING SAID BRAID WITH A RESIN MATERIAL;CURING SAID RESIN TO BOND THE THREADS OF SAID THREADS TOGETHER AND TOSAID INNER SLEEVE; AND REMOVING SAID MANDREL FROM SAID INNER SLEEVE.